A Chinese Utility Model patent application number CN 201320549548.2 entitled STEREOSCOPIC PROJECTION DEVICE WITH LOW THROW RATIO AND HIGH LIGHT EFFICIENCY AND STEREOSCOPIC PROJECTION SYSTEM, discloses a 3D projection light modulation device with high light efficiency, comprising essentially a polarization beam splitter assembly consisting of three isosceles right-angled prisms attached together. FIG. 1A shows two polarization beam splitting planes relative to a horizontal center line of an incident plane of the prism. When ideal parallel light is incident perpendicularly on the polarization beam splitter assembly from the patent CN 201320549548.2, light ray can be emitted normally. The reflected light ray close to the joint seam is reflected upward and downward at an angle almost parallel to the incident plane and is emitted by the upper and lower surfaces of the polarization beam splitter assembly. In a normal state, the light ray is reflected twice, by a polarization beam splitting coating and a reflective mirror, respectively. However, no lens of any digital film projector emits parallel light, and when a light ray is incident obliquely on the polarization beam splitter assembly, the light ray striking at the joint seam at the center of the polarization beam splitting prism assembly is reflected by a 45-degree polarization beam splitting surface and then arrives at the incident plane of the polarization beam splitting prism assembly. Since this surface is an interface between glass and air, light rays incident on said interface at a large angle will be totally reflected. As shown in FIG. 1B, these reflected light rays are reflected by the reflective mirror, and then arrive at the screen and finally form images. In this case, the light rays have been reflected three times in total, by the polarization beam splitting coating, the incident plane, and the reflective mirror, respectively, so that the incident light ray falling close to the joint seam at the center of the polarization beam splitter assembly eventually forms an additional inversed image on the screen, thereby causing severe impact on the image quality of the screen.
The prism configuration adopted by the present invention can avoid this technical defect presented in the technical scheme of the patent CN 201320549548.2.